Update panic message to be clearer about env-vars

Esteban Kuber requested that the panic message make it clear
that `RUST_BACKTRACE=1` is an environment variable.  This change
makes that clear.  Wording provided in part by David Tolnay.

1 files changed, 2 insertions, 1 deletions

diff --git a/src/libstd/panicking.rs b/src/libstd/panicking.rs
index 6b47ba6d1cb..b6180fbeb50 100644
--- a/src/libstd/panicking.rs
+++ b/src/libstd/panicking.rs
@@ -209,7 +209,8 @@ fn default_hook(info: &PanicInfo) {
             if let Some(format) = log_backtrace {
                 let _ = backtrace::print(err, format);
             } else if FIRST_PANIC.compare_and_swap(true, false, Ordering::SeqCst) {
-                let _ = writeln!(err, "note: Run with `RUST_BACKTRACE=1` for a backtrace.");
+                let _ = writeln!(err, "note: Run with `RUST_BACKTRACE=1` \
+                                       environment variable to display a backtrace.");
             }
         }
     };